This invention relates to instant-on light sources and, more particularly, to light sources wherein a discharge lamp is combined with incandescent filaments and a solid state ballast to provide instant-on lighting.
High pressure metal vapor discharge lamps provide significantly higher efficiencies than incandescent lamps and are widely used for general lighting purposes. Such lamps can be operated from standard 60 Hz ac power but require ballast circuits for power factor correction and voltage step-up to develop the requisite high starting potential. Typical ballast circuits include a capacitor and an inductor and are large and heavy. In addition, conventional ballast circuits provide poor power regulation for line-voltage variations, are expensive, cause flicker at 60 Hz, and often introduce an audible humming noise into the environment.
An inherent disadvantage of metal vapor discharge lamps is the warm-up period of several minutes during which only a low level of illumination is available. The warm-up period or cold-start delay is due to the necessity for the metallic fill material to be vaporized and the lamp envelope to be warmed up before full light output is attained. Furthermore, when power to the lamp is momentarily interrupted, the discharge is extinguished and cannot be re-initiated until the lamp cools off and the pressure in the lamp is reduced. After the discharge is re-ignited, the warm-up period described above must be repeated before the lamp again reaches full light output. The hot restart delay is thus longer than the cold-start delay. The delays involved in starting and restarting metal vapor lamps are unacceptable in some applications, for example, in household lighting.
It is known to use standby incandescent filaments to provide illumination during the cold-start delay and hot restart delay period associated with metal vapor discharge lamps. Various arrangements have been shown in the prior art for operating an incandescent filament during cold start and hot restart periods of a discharge lamp operating at 60 Hz in conjunction with a conventional inductive ballast.
Metal vapor arc lamps have been operated at frequencies well above 60 Hz. A metal vapor arc lamp operated at 20 to 50 KHz is disclosed in U.S. Pat. No. 4,151,445 issued Apr. 24, 1979 to Davenport et al. A comparator circuit senses when the arc lamp voltage is outside a predetermined range and turns on a standby incandescent filament when the arc lamp voltage is outside that predetermined range. While the disclosed light source provides instant light output, it disadvantageously utilizes relatively heavy and expensive inductive components and it generates radio frequency interference (RFI), especially during lamp start-up, thereby potentially causing electromagnetic interference with other electronic equipment.
Metal vapor discharge lamps can be operated directly from dc power sources. Solid state dc lamp ballasts utilizing switching regulators are described by P. Schiff in "Solid-State Ballasting of Mercury-Arc Lamps" RCA Application Note AN-3616, November 1973. A solid state dc lamp ballast, commercially available from Quietlite International Ltd. under Model No. QL-75-4, utilizes a series regulator. The discharge lamp draws current from the series regulator during warm-up through a series resistor which is electronically bypassed after the discharge lamp reaches operating temperature. The problem of cold-start delay and hot restart delay remains when the discharge lamp is operated by dc power.
Solid state dc ballasts provide power regulation and eliminate heavy magnetic components, 60 Hz flicker, electromagnetic interference, and audible humming. Furthermore, the metal vapor discharge lamp is relatively efficient. Therefore, it is desirable to provide a solid state dc ballasted light source which has light output during the cold-start and hot restart periods of the discharge lamp. Such a light source is a suitable replacement for the incandescent lamp.